In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added gaseous products from lower-fuel-value carbonaceous feedstocks, such as petroleum coke and coal, is receiving renewed attention. In general, such carbonaceous materials can be converted to a plurality of gases, such as a gas mixture comprising carbon monoxide, hydrogen and methane, by the gasification of the material at elevated temperatures and pressures.
The production of gas mixtures comprising carbon monoxide and hydrogen from coal and other carbon sources is generally known in the art. This type of gasification typically takes places at higher temperatures and pressures as disclosed, for example, in U.S. Pat. Nos. 4,251,227, 4,252,736, and 6,114,400.
Gasification of such materials to favor production of methane is usually a catalytic process that takes place as slightly more moderate temperatures and pressures as disclosed, for example, in U.S. Pat. Nos. 3,828,474,3,998,607, 4,057,512, 4,092,125, 4,094,650, 4,204,843, 4,468,231, 4,500,323, 4,541,841, 4,551,155, 4,558,027, 4,606,105, 4,617,027, 4,609,456, 5,017,282, 5,055,181, U.S. Pat. No. 6,187,465, U.S. Pat. No. 6,790,430, U.S. Pat. No. 6,894,183, U.S. Pat. No. 6,955,695, US2003/0167961A1, US2006/0265953A1, US2007/000177A1, US2007/083072A1, US2007/0277437A1, US2009/0048476A1, US2009/0090056A1, US2009/0090055A1, US2009/0165383A1, US2009/0166588A1, US2009/0165379A1, US2009/0170968A1, US2009/0165380A1, US2009/0165381A1, US2009/0165361A1, US2009/0165382A1, US2009/0169449A1, US2009/0169448A1, US2009/0165376A1, US2009/0165384A1, US2009/0217584A1, US2009/0217585A1, US2009/0217590A1, US2009/0217586A1, US2009/0217588A1, US2009/0217589A1, US2009/0217575A1, US2009/0217587A1 and GB1599932. Even in processes that favor methane production, appreciable quantities of carbon monoxide and hydrogen are produced as well.
In both types of gasifications, fine unreacted carbonaceous materials are removed from the raw gases produced by the gasifier, the gases are cooled and scrubbed in multiple processes to remove undesirable contaminants and other side-products including carbon dioxide and hydrogen sulfide.
The reaction of carbon monoxide and hydrogen to produce methane has been carried out in the presence of a catalyst such as various iron, nickel or cobalt compounds. Although these methanation catalysts are reasonably effective, such materials are highly sensitive to sulfur compounds and are quickly poisoned. To avoid this and continue use of these catalysts, the gases fed to the methanation reactor must be treated to remove both organic and inorganic constituents containing sulfur, generally by first scrubbing the gas stream with a solvent such as methanol to eliminate most of the hydrogen sulfide and mercaptans, and then removing the last traces of these impurities by adsorption on reduced zinc oxide or a similar adsorbent. These catalysts also require periodic regeneration. These feed gas purification and catalyst regeneration steps are expensive and time consuming.
The development of carbon-alkali metal based methanation catalysts, such as disclosed in U.S. Pat. No. 3,958,957, has provided for more robust catalysts than the iron, nickel and cobalt catalysts. These carbon-alkali catalysts are resistant to high temperature sintering and catalyst degradation, as well as resistant to poisoning by sulfur compounds. Nevertheless, these carbon-alkali catalysts require the sacrifice of carbon material as well as alkali compounds that could otherwise be used as catalyst loaded carbonaceous feedstock in various gasification processes.
Therefore, a need remains for improved methods for producing a methane-enriched gas from gas mixtures comprising carbon monoxide and hydrogen, and alternative methanation catalysts for use in such processes.